In order to improve the performance of single-user MIMO (Multiple Input Multiple Output) and multi-user MIMO, Alcatel-Lucent Company etc. have invented multi-BS (base station) collaborative manner. Its basic idea is the joint MIMO transmission and receiving among multiple BSs and one or more mobile stations (MS) on the same wireless resources. This technology has basic features in two aspects:
1) Multiple BSs can serve one or more MSs on the same time-frequency resources via the coordination among the BSs.
2) Each BS can serve one or more MSs on the same time-frequency resources.
In the multi-BS collaborative MIMO manner, each BS needs to set up the best collaborative relationship and design the optimal MIMO transmission scheme according to the channel information of all the served MSs. Obtaining downlink channel information can be realized in different manners:
i. For TDD (Time-Division Duplex) system, the BS can obtain the uplink channel fading response through uplink sounding signal or uplink traffic channels because of the reciprocity between the uplink channel and the downlink channel of TDD system. The uplink channel estimation can be used for approximating the channel state information of the downlink as long as the time interval between the uplink and downlink transmission is smaller than the coherence time.
ii. For FDD (Frequency-Division Duplex) system, the downlink channel fading response is almost uncorrelated with the uplink one, since the frequency interval between the uplink channel and the downlink channel generally exceeds the channel coherent bandwidth. So it is impossible to estimate the channel state information of the downlink based on the uplink channel response.
Therefore in FDD system, the channel state information can be obtained through the manner in which each MS feeds back the downlink channel coefficients to BS; or the downlink channel information can be obtained through the manner in which each MS calculates multiple codebooks and chooses the best one therefrom, then feeds back the index and SINR (signal-to-interference and noise ratio) of the channel to BS.
The drawback of the described manner i is that it is only applicable to TDD system and not applicable to FDD system;
And the drawback of the described manner ii is that the amount of the feedback of the terminals is huge and will occupy the limited uplink channel resources in the manner in which the downlink channel coefficients are feedback; and in the manner in which the codebooks are calculated, the computation complexity of the terminals is higher.